Robust multi particle system for color electrophoretic displays with very low driving voltages comprising a low amount of electrolytes

Abstract

An electrophoretic dispersion with at least two particle species is proposed in which a minimal number (and concentration) of free compounds is present in solution. All chemicals needed to color and stabilize the pigment particles are preferably covalently attached to or within the particles. This enhances the robustness of the system and minimizes the conductivity of the dispersion so that the driving voltage for an electrophoretic display is reduced.

The invention claimed is: 
     
       1. Electrophoretic dispersion comprising a first polymer particle species having a first color and a first refractive index and a second polymer particle species having a second color and a second refractive index, wherein each particle species contains one or more charge forming groups, wherein the charge forming groups are bonded to the particle species either by covalent bonding or by non-covalent adsorption or bonding, and wherein the first particle species comprises an ionisable surface group that can be ionized by reduction or oxidation so that the first particle species is capable of dissociating into a first ionic species and a first charged particle species. 
     
     
       2. Electrophoretic dispersion according to  claim 1 , wherein non-covalent adsorption comprises physisorption or chemisorption, preferably irreversible physisorption or chemisorption. 
     
     
       3. Electrophoretic dispersion according to  claim 1 , wherein the dispersion contains less than 10%, preferably less than 5, more preferably less than 1%, even more preferably less than 0,1% of other free electrolytes (and other (uncharged) additives) compared to the total amount of particle species. 
     
     
       4. Electrophoretic dispersion according to  claim 1 , wherein the ratio between the total amount of positive charges and the total amount of negative charges, expressed as N+*Q+: N−*Q−, is between 0.9 and 1.1, preferable between 0.99 and 1.01, more preferably between 0.999 and 1.001 even more preferably between 0.9999 and 1.0001, wherein
 −N+: particle concentration of positively charged particles; 
 −N−: particle concentration of negatively charged particles; 
 −Q+: particle charge of positively charged particles; 
 −Q−: particle charge of negatively charged particles. 
 
     
     
       5. Electrophoretic dispersion according to  claim 1 , wherein more than 90%, preferably more than 95%, more preferably more than 99% and even more preferable more than 99,9% of the positive and negative charges in the dispersion is located on the particle species. 
     
     
       6. Electrophoretic dispersion according to  claim 1 , wherein the dispersion further comprises a charge transfer agent, preferably in an amount of less than 10%, preferably less than 5, more preferably less than 1%, even more preferably less than 0,1%, compared to the total amount of particle species. 
     
     
       7. Electrophoretic dispersion according to  claim 1 , further comprising a dispersion medium selected from the group consisting of saturated and/or unsaturated, branched or unbranched, optionally hetero-atom (S,O,N etc.) containing, substituted or unsubstituted (halogens, nitro, amine etc.), aliphatic or (hetero)aromatic hydrocarbon solvents or (per)fluorinated hydrocarbons or (per)fluorinated polyethers, preferably from the group consisting of tetrachloorethane, carbon tetrachloride, pentane, octane, decane, cyclohexane, benzene, toluene, xylene, undecane, dodecane, tetradecane, Isopars, chloroform sec-butylbenzene, Fluorinert™and the like, or a mixture thereof. 
     
     
       8. Method for the preparation of an electrophoretic dispersion comprising admixing particle species comprising ionisable surface groups as defined in  claim 1 , preferably in a suitable dispersion medium. 
     
     
       9. Method according to  claim 8 , wherein the admixed particle species comprising ionisable groups act as charging agents for each other. 
     
     
       10. Electrophoretic dispersion according to  claim 1 , wherein a charge transfer agent is present, preferably a non-ionic charge transfer agent, more preferably a sterically hindered amine or ether. 
     
     
       11. Electrophoretic display comprising a dispersion as defined in  claim 1 . 
     
     
       12. Electrophoretic dispersion according to  claim 1 , wherein each particle species further contains one or more stabilisers, wherein the stabilisers are bonded to the particle species either by covalent bonding or by non-covalent adsorption or bonding. 
     
     
       13. Electrophoretic dispersion according to  claim 1 , wherein the second particle species is capable of binding the first ionic species to provide a second charged particle species. 
     
     
       14. Electrophoretic dispersion according to  claim 1 , wherein the first ionic species is capable of providing a charge to the second particle species to provide a second charged particle species. 
     
     
       15. Electrophoretic dispersion according to  claim 1 , wherein a first charged particle species is obtained by dissociation of the first particle species in a first ionic species and the first charged particle species, and wherein an oppositely charged second charged particle species is obtained by incorporation of the first ionic species in the second particle species.